This invention relates to a method for recycling waste products. More particularly, this invention relates to a method for the recycling of papermaking sludge produced as waste material in the manufacture of paper, cardboard, tissue and related products. This invention relates to a recycling method for such papermaking sludge, producing thereby waste much reduced in bulk for more effective disposal in landfills; waste much reduced in potential toxicity; waste material useful as component materials in the manufacture of cement block, brick and other products; waste material useful as an additive and extender in gravel, backfill and other operations.
This invention is a modification and improvement of the method for reducing and recycling papermaking sludge described and claimed in U.S. Pat. No. 5,018,459.
Paper manufacturing processes typically begin with a slurry of pulp and water produced as output from a separate pulping process. This pulping process itself produces many kinds of waste materials, not the subject of the present invention. The pulp slurry is typically delivered to the papermaking facility, additional chemicals may be added, and the resulting pulp and water slurry made into paper by means of a variety of processes, typically involving rapid withdrawal of the water through a screen, depositing thereby the paper onto the screen. The waste water is reprocessed to recover as much usable material and reusable water as economically feasible. Typically, the water is sent to a "settling tank" in which heavier components settle to the bottom of the tank and are drawn off as "sludge". This sludge is typically processed further to remove additional water (typically by mechanical pressing), leading to a quantity of sludge for final disposal. This sludge is known in the industry by a variety of names, including: "primary waste treatment sludge", "paper mill sludge", "process residual", "waste treatment sludge", "waste treatment plant (`WTP`) sludge", to name a few. For simplicity, we will use the term "papermaking sludge" to denote this product, or merely as "sludge". The reprocessing and recycling of this papermaking sludge is the subject of the present invention.
The manufacture of paper, cardboard and related products typically results in large quantities such papermaking sludge. As an approximate rule-of-thumb, 20% of the tonnage of paper products produced by a given paper mill will be produced as sludge waste by-product. Thus, larger paper mills can easily produce in excess of 500 tons per day of sludge requiring disposal.
This papermaking sludge contains large amounts of water, wood fibers, calcium carbonate, other minerals and clays, various mixing catalysts (typically soy protein or casein), and chlorine-based purifying agents used in the paper making process. There is no precise composition for this sludge because there are substantial variations in the wood and other feedstocks used; in the processing materials which must be used to make different types of paper products; and even considerable variation in the processes used by different paper makers in making similar products.
The disposal of this papermaking sludge is a continuing problem for the paper manufacturers and for the environment in general. Current practice is to subject the sludge to a mechanical pressing operation to remove excess moisture. The resulting residue still holds large quantities of water, as well as the other materials listed above. Under current procedures, this residue is then typically deposited into a landfill for indefinite storage.
There are several problems associated with this treatment of papermaking sludge. First of all, it is getting increasingly difficult to locate suitable landfills as existing landfills become full and residents often oppose expansion or creation of new landfills near residences. In addition, the materials which leach from sludge deposited into landfills is hazardous itself. This increases the cost of engineering a suitable disposal site (due to the increased costs of handling such leachates), and increases community and environmental concerns.
The present invention proposes an alternative treatment for this papermaking sludge, as a modification and improvement of that described in U.S. Pat. No. 5,018,459. The present invention markedly reduces the bulk of the sludge to be disposed (by typically 75%), thereby markedly extending the life of disposal landfills. Also, the present invention produces a material for disposal that is much less susceptible to leaching, thereby reducing the potential environmental hazards for landfills.
In addition, the present invention offers the possibility of dispensing with landfills entirely. The present invention demonstrates how papermaking sludge can be processed into a commercial product similar to re-calcified calcium carbonate. (Due to the variation in composition of the sludge noted above, the processed sludge from the present invention has varying compositions.) This processed sludge has many of the useful properties of calcium carbonate and finds use in asphalt, asphalt sealers and coatings, concrete block and pipe, other brick and structural pipe, for gravel-based construction operations such as parking lots or highways, as an additive to backfill material for sewer water and gas pipelines, in ceramics and many other applications.
U.S. Pat. No. 5,018,459 describes a one-step reduction and recycling process by use of a rotary kiln. The rotary kiln has the effect of placing the sludge into suspension. While in suspension, the combustible components of the sludge dry then ignite, thereby adding their heat energy to the process. Thus, the external energy (typically natural gas) required by the process of U.S. Pat. No. 5,018,459 is markedly reduced by the use of rotary kilns.
The present invention represents an improvement of U.S. Pat. No. 5,018,459 in that aggregate is added to the process along with the sludge. The use of aggregate in combination with sludge in a rotary kiln seems to have at least four beneficial effects. 1) The rotation of the aggregate along with the sludge helps keep the sludge in suspension, adding to the efficiency of mixing and combustion. 2) Heated aggregate retained in the rotary kiln serves to increase heat transfer to the sludge, tending to pre-heat and partially dry sludge before the sludge reaches the hot zone of the kiln. 3) Certain aggregates (such as limestone or limestone-containing aggregate) serve to adjust the pH of the process towards alkaline. 4) Some evidence suggests that proper pH control of the process tends to hinder or suppress the formation of dioxins and furans. Aggregate is commonly used in rotary kilns as a means for promoting suspension of materials otherwise prone to agglomeration (sometimes referred to as a mixing "catalyst" although the effect is primarily mechanical-to promote mixing-rather than to catalyze chemical changes.) However, we are aware of no use of aggregate, prior to the present invention, in rotary kiln recycling of papermaking sludge, having the beneficial effects noted above.
Several other approaches have been tried for the disposal of papermaking sludge. These generally fall into two catagories: simple incineration; complex, multistep processing, typically attempting to extract higher value materials from the sludge.
There have been several approaches to the disposal of papermaking sludge involving complex, multistep processes. Such processes typically involve attempts to extract additional usable materials (like titanium) from specialized types of papermaking sludge. Of necessity, such processing involves numerous steps, some of them quite complex and expensive in their own right. Typical examples include the work of Goto (Aug. 12, 1981, S. Goto, "Use of Paper Sludge for Cement Manufacture", Unexamined Japanese patent application no. 2905-1908, disclosure no. 100222-1981). This work uses a hot dryer (rather than an incinerator) for preliminary processing of papermaking sludge to reduce it to a cake. This cake is then pulverized mechanically with a rotating fork-like machine to make small particles such that 70% of the total have diameters 0.5 mm or less. The resulting small particles are burned, perhaps in a rotary kiln or fluidized bed incinerator. Missing the essential simplicity (and, therefore, the economy) of the present invention, the Goto work has not had a significant effect on eliminating sludge from landfills.
Another typical approach is illustrated by the study published by the S. D. Warren Co. (1971, July, S. D. Warren, Co., "Sludge Material Recovery System for Manufacturers of Pigmented Papers," US Environmental Protection Agency Report W72-11605). A study is reported investigating processes by means of which waste sludge from papermaking might be used to produce a product which can be reused as a component of papermaking once again. This study has a very complex process in which: 1) Gritty materials first are removed from sludge (the present process has no such restriction); 2) The waste sludge to be processed must have high inorganic pigment content and low fibrous content (the present process prefers high fiber content to add to the heat of combustion but is quite feasible with lower fiber contents). 3) A multistep process then follows, with sludge dilution, consistency regulation, centricleaning, vacuum filtering, shredding, drying, followed by processing in rotary kiln. 4) The resulting product is used back into paper. Typical of such complex, multistep processes, this study concludes that high capital cost makes this process "not economically feasible" at this time (Conclusion #8, page 1). The essence of commercial feasibility is often simplicity. The present invention has one of its foundations in the essential simplicity of the process, previously overlooked, or obscured by attempts to extract other products from sludge.
The other broad class of attempts to dispose of papermaking sludge involves simple combustion of the sludge in a bark-burning boiler, hog fuel burner, or other incinerator. While disposing of sludge, these techniques typically require large amounts of additional process fuel, thereby drastically reducing the economics of the process and erecting serious barriers to practical commercial use.
In contrast, the present process (and the U.S. Pat. No. 5,018,459 process), using a rotary kiln, require only moderate amounts of gas and render the process economically feasible. Indeed, the present process typically is run with a large natural gas flame until the sludge is in suspension and adding its heat energy to the process. Once this occurs, the natural gas flame is reduced and the process becomes almost completely self-sustaining. On a simple heat balance, for each pound of sludge, combustion of the wood fibers typically generates approximately twice the heat necessary to drive off the water from a similar pound of sludge. Therefore, under theoretically ideal conditions, the papermaking sludge would be a net fuel source while producing commercially useful ash as a combustion product. In practice, however, additional heat has been required to make the process occur continuously in a practical way. Nevertheless, it is common to be able to reduce the initial gas flame to only 10% (or less) of its initial gas consumption once sludge heat joins the process. Therefore, in continuous operation the start-up energy consumption becomes negligible and the rotary kiln process of the present invention is markedly energy-efficient.